Screw supports of this kind primarily serve to counteract tendencies of the screw, which usually is a ball screw, to start vibrating and bending or being deflected outwards, when the screw reaches a critical rotational speed. Such lateral deflections depend among other things on the length of the screw and its rotational speed. When the screw is long and/or rotates at a high speed, this critical rotational-speed value is reached at an earlier stage than when the screw is shorter and/or rotates at a lower speed.
In the case of long linear units having a length of several meters, up to six meters, and being fitted with a long screw of a corresponding length, such tendencies to deflection outwards of the screw may be counteracted with the aid of two or more such screw supports. To allow the carriage to be slid from one end of the linear unit to the opposite one, various driver means have been suggested to displace the screw supports from a first end stop position along the longitudinal extension of the screw to another end stop position and then, in a return displacement movement, return the screw supports to their first end stop position.
A prior-art driver means configured as interacting locking means between carriage and screw supports suffers from the disadvantage of being unnecessarily complicated and in addition exposed to excess wear from the displacement of the screw supports backwards and forwards between the end stop positions by means of the carriage, a feature which often leads to unsatisfactory operational reliability and stability of the linear unit.
Often, such interacting locking means also require so much space in the area around the linear unit that there is a risk that, while moving, the screw supports hook onto some component of the linear unit, which likewise impairs the operational reliability and stability of the linear unit and may lead to breakdowns. In addition, interacting locking means between carriage and screw supports of a kind that are constantly interconnected and disconnected mechanically generate a strong percussion-like noise, which often is felt as most disturbing.
In another prior-art and much improved driver means that eliminates entirely the need for interacting locking means acting between carriage and screw supports, the screw and the screw support are fitted with interacting driver means which, upon rotation of the screw in one direction, carry with them the screw support, displacing it from one of its stop end position to the opposite one and reversely, and which means slide on top of the screw in the end stop positions of the screw support until the screw changes its direction of rotation.
Irrespective of the length of the linear unit and the type of the driver means therein, the screw ends are mounted to ensure that they are centered in their associated bearings. Because of such factors as inevitable manufacturing and matching tolerances and wear on the one hand between screw and screw support and on the other between screw support and profiled rail, play and misalignment of the screw supports relative to the screw and the profiled rail often arise, particularly when the linear unit has been in use for some time. In turn, this leads to the screw supports being exposed to uneven loads from the screw during their movement backwards and forwards along the profiled rail, usually because the screw, when rotating, constantly or alternately exerts pressure on one or the other of the sides of the through-bore formed in the screw support, with consequential increase of wear and noise and in the end functional disturbances and even breakdown.